Subsea oil and gas production wells are sealed off from the environment using apparatus known as a production xmas trees. Two types are commonly used; firstly the vertical xmas tree, which has gate valves in the production bore allowing isolation of the well through the remote operation of these valves, and secondly, the horizontal or “spool” xmas tree in which the production bore is sealed off using plugs run into a tubing hanger installed inside the tree.
Increasingly, horizontal type xmas trees are being used with subsea wells for reasons of cost and convenience. Horizontal xmas trees allow wells to be brought into production in less time and at a lower cost than vertical xmas trees. Furthermore, horizontal xmas trees allow for more modern and larger bore completion systems to be used in the well.
However, significant problems have been encountered when trying to correctly set plugs in the tree, or when trying to recover plugs from the tree. For example, the plug can be set in similar, but incorrect profiles within the tree resulting in low integrity sealing of the well. Secondly large forces are required to set or retrieve the plug from the tree, and thirdly there is no positive indication of the plug being correctly set.
Since the plugs are primary barriers in isolating the high pressure hydrocarbon well fluids from the environment, it is critical they are correctly placed and fitted within the xmas tree system.